The design of engines that travel at supersonic speeds (e.g. possibly in the range of Mach 2 to Mach 3 at cruise, or possibly higher) involves a number of problems similar to those encountered in the design of subsonic jet engines. For both supersonic and subsonic engines, there are the generally concerns regarding weight, size, complexity, reliability, cost, etc., and also concerns relative to performance (e.g. thrust, specific fuel consumption, etc.). However, supersonic jet engines pose some special problems. More particularly, present date optimized designs for supersonic turbo-jet type engines are characterized in that these have relatively high jet velocities, and thus create a high level of noise. Noise suppression in this type of engine is one of the most critical technical problems to be solved in making an environmentally acceptable commercial supersonic jet transport. Another consideration is that the supersonic jet engine must be designed to function adequately through a broad range of operating modes (i.e. take-off and climb, acceleration up to supersonic cruise Mach number, as well as being able to cruise at both subsonic and supersonic speeds).
With regard to noise suppression, in the last several decades, there have been many different systems proposed and/or used for suppressing noise. One general approach has been to mix the higher velocity jet exhaust with lower velocity air, and there are many patents and other technical disclosures relating to variations of this basic concept. Further, there is use of noise suppressing structural materials, along with the use of panels deployed in the gaseous outflow path from the engine. However, quite commonly, these noise suppressing devices will degrade performance for modes of operation (e.g. supersonic cruise) not requiring this degree of noise suppression.
A search of the patent literature has disclosed a number of patents, and these are given below:
Hache et al U.S. Pat. No. 3,910,375 discloses what is called a "jet engine silencer" where there is an ambient air induction system which feeds into the center body of the engine. Within the center body there are chutes which are normally stowed, and for suppressor operation they are deployed so that these emit the ambient air with the primary exhaust to create more rapid mixing. The actual mixing occurs aft of the exit plane of the nozzle.
Bonncaud et al U.S. Pat. No. 3,820,626 discloses what can be referred to as a "spade-type" sound suppressor. For noise suppression, members are moved into the exhaust stream to create obstructions and turbulence in the stream to enhance mixing of the exhaust stream with ambient air at a location aft of the nozzle.
Straight U.S. U.S. Pat. No. 3,780,827 shows an exhaust nozzle where there are hollow struts which carry air into a sting at the center of the engine. This air in the sting exits at the nozzle location, serving as a low velocity inner core of secondary air to provide noise reduction.
Colville et al U.S. Pat. No. 3,352,494 shows what is called a supersonic jet propulsion nozzle. This has a collapsing center body plug nozzle with a translating shroud. Noise suppression is obtained by deploying flap members 52 into he gaseous exhaust to promote mixing with ambient air.
Young U.S. Pat. No. 3,161,257 discloses a nozzle silencing system which deploys elements into the air stream to cause a swirling motion, and thus cause mixing with ambient air at a location aft of the nozzle end opening.
Brown U.S. Pat. No. 2,987,879 discloses a co-annular nozzle where ambient air is taken in through radially extending struts to the center body of the engine. Then for noise suppression, this air is moved into the middle portion of the exhaust stream to increase the diameter of the jet stream so that it mixes more quickly with the outside air. This achieves its noise reduction by what is sometimes called a "co-annular" reduction benefit.
Glenn U.S. Pat. No. 2,931,169 discloses what is called a "variable convergent/divergent exhaust nozzle" which is intended to lower drag and improve the thrust of the nozzle over a wide flight regime from subsonic to supersonic, and has been used in supersonic jet fighter aircraft over a number of years. This does provide a noise reduction benefit to some extent since the air enters through an ejector and mixes with the primary nozzle. This essentially comprises an annular ring of air which flows around the main primary exhaust.